Project Summary Osteoarthritis (OA) represents an enormous unmet medical need. Fifty years of intense cartilage- and bone- centered research into the pathogenesis of OA are yet to yield therapies that can halt the progression of structural joint damage and reduce the associated chronic pain. We propose the paradigm-shifting hypothesis that symptomatic OA is not fundamentally due to structural degeneration of joint tissues but rather is primarily a disease driven by loss of joint sensory innervation. Our ?neurocentric? hypothesis asserts that the intra-articular (i.a.) sensory innervation has critical protective effects on joint health and that age-related loss of sensory innervation renders the joint vulnerable to OA. We will employ state-of-the-art neurobiological techniques to unambiguously accept or reject this hypothesis, and dissect the individual role of two distinct subsets of sensory neurons, nociceptors and proprioceptors, in joint health. We will study both spontaneous OA and post- traumatic OA (PTOA). To study PTOA, we will use a validated murine model, destabilization of the medial meniscus (DMM) that captures the long-term progression of OA and provides a unique opportunity to test the central hypothesis. In the R61 phase, a focused set of experiments will test the hypothesis that i.a. sensory innervation has an ongoing trophic effect on the knee, using complementary approaches in 2 aims. Aim 1 will test the hypothesis that ablation of sensory afferents in the knee drives the onset of OA. Irreversible ablation of i.a. joint afferents will be achieved through i.a. administration of diphtheria toxin (dtA), targeted to either nociceptors (using NaV1.8 as a marker) or proprioceptors (using parvalbumin (PV) as a marker). We will study the effect of knee-specific ablation on onset and progression of age-related and PTOA (DMM). Selective administration of dtA to populations of knee afferents will be achieved using specifically engineered floxed AAV constructs injected into the knee joints of NaV1.8-cre or PV-cre mice. Aim 2 will test the hypothesis that chronic chemogenetic inhibition of sensory afferent activity within the knee drives the onset of PTOA. Reversible inhibition of joint afferent electrical activity will be achieved through DREADD technology (Designer Receptor Exclusively Activated by Designer Drugs), targeted to nociceptors or proprioceptors. Specific expression of the inhibitory DREADD receptor, hM4Di, in subsets of sensory afferents will also use a strategy based on floxed AAV injection into the knees of NaV1.8-cre or PV-cre mice. If we are able to accept our hypothesis, year 3 will focus on consolidating and fine-tuning the hypothesis, and start mechanistic exploration. This will include testing the hypothesis that exciting sensory neurons with stimulatory DREADDs will protect from OA, identifying which subsets of NaV1.8 nociceptors are critical for maintenance of joint health, and pursue mechanistic studies on the role of i.a. proprioceptors. We anticipate that these studies will suggest novel targets for disease modifying therapeutic interventions, and will aid greatly in the development of safer analgesics by elucidating the complex interplay between sensory nerves and joint health.